PURPOSE: To clean the head surface of an optical sensor in a state where it is disposed and to always realize measurement of high accuracy as the result by arranging an ultrasonic oscillation part at an adjacent position to the contacting surface of the optical sensor coming into contact with liquid.

CONSTITUTION: A measuring system is housed in a case 10 in water-tight construction and simultaneously the ultrasonic oscillator 11 is juxtaposed by being adjacent to a prism 1. The prism 1 is exposed from the case 10 so as to make the head surface come into contact with the liquid to be measured 6. The oscillator 11 is also provided so as to face the ultrasonic radiating surface thereof to the liquid 6. The case 10 is set in such a state that it is soaked in the liquid 6. Besides, the oscillator 11 is also desired to be disposed by exposing it from the case 10 so as to face the ultrasonic radiating surface thereof to the head surface of the prism 1. By regularly driving the oscillator 11 in such a using state, the head surface of the prism 1 is ultrasonic-cleaned. Since the oscillator 11 can be driven by remote manipulation and mechanical operation is not needed, the easy and precise cleaning is executed.